Motor 46 Trip: Causes and Diagnosis

Short answer

The motor 46 measures negative-sequence current, which heats the rotor with currents at twice the supply frequency. Because the motor negative-sequence impedance is low, a small voltage unbalance produces a current unbalance several times larger. When it trips, the most common causes are an open phase upstream, a grid unbalance larger than expected, or a wiring or setting error in the phase rotation and the CTs.

Symptom

A motor protection relay trips on unbalance, negative sequence or phase loss (depending on the manufacturer: 46, unbalance, current unbalance or phase loss). The motor is checked and has no visible damage. Sometimes the trip repeats at a certain time of day, when another large motor starts, or on all the motors of the same bus at once.

The negative-sequence protection 46 exists because negative-sequence current creates in the air gap a field that rotates in the opposite direction to the rotor. That field induces in the rotor bars currents close to twice the supply frequency, which because of skin effect meet a higher resistance and heat the rotor much more than the same positive-sequence current. For that reason many motor thermal models weight negative sequence with a larger factor than positive, as explained in thermal overload 49.

The other particularity of the motor is its negative-sequence impedance, similar to the locked-rotor one and much lower than the impedance it shows to positive sequence while running. The motor therefore amplifies unbalance: a small voltage unbalance on the bus becomes a current unbalance several times larger. A 46 trip does not mean the motor is damaged. It means its supply is not symmetrical, and the task is to find out why.

Possible causes, from most to least likely

Cause	Why it happens	What gives it away
Open phase upstream	Blown fuse, contactor or breaker pole that did not close, burned terminal, broken conductor	One current near zero and the other two high and opposite
Grid voltage unbalance	Single-phase loads, transformer with a different tap per phase, high-resistance connections	Unbalanced currents on all motors of the bus
Phase rotation wrongly set or CTs swapped	Error at commissioning or after rewiring	High I2 with a balanced load from the first energization
CT ratio or connection different between phases	CT of another ratio, wrong tap, loose terminal	One current systematically different with a healthy motor
Stator fault	Incipient turn-to-turn or phase-to-phase short circuit	Unbalance that grows with time; may come with a ground or differential trip
External fault or nearby single-pole reclose	Transient negative sequence in the grid	Trip coincident with a grid fault
Setting too sensitive for the real grid	Pickup below the normal unbalance of the site	Trips during hours of unbalanced load, with no other anomaly

What to check first

Currents and voltages of the three phases in the relay record at the moment of the trip, in magnitude and angle. Calculate I2/I1 and V2/V1.
Comparison with other motors on the same bus. If all see unbalance, the cause is in the supply; if only one, it is in its circuit or in the motor.
Fuses, contactor and breaker of the starter: condition of each pole, terminals and connections. A thermography survey under load locates high-resistance connections.
Phase rotation setting of the relay and CT connection, especially if the motor is new or was worked on.
Grid voltage at the motor bus, with a recorder, over several days if the trip is sporadic.

First screening of a motor 46 trip

Was one of the three currents near zero?
- Yes: Open phase: fuse, contactor or breaker pole, terminal or conductor
- No: Do other motors on the same bus record unbalance?
  - Yes: Grid voltage unbalance: measure the voltage at the bus
  - No: Does the unbalance appear with a balanced load since commissioning?
    - Yes: Phase rotation wrongly set or CTs swapped
    - No: Review the motor circuit and stator: connection, CT or incipient fault

How it is confirmed

Bus voltage measurement with a power quality analyzer and calculation of the voltage unbalance. See power quality analyzer.
Primary current measurement with a clamp on each phase, compared with what the relay measures.
Motor tests if a stator fault is suspected: winding resistance per phase, insulation and polarization index and, on medium-voltage motors, surge tests. See medium-voltage motor tests.
CT verification and phase rotation setting check with secondary injection.

How to keep it from repeating

Set the 46 with the real unbalance of the site and the motor capability stated by its manufacturer, within a study that considers the grid.
Monitor voltage unbalance on buses with many motors, with an alarm before the trip.
Starter maintenance: fuses, contactor poles and terminals, with periodic thermography.
Verify rotation and CTs at commissioning with the motor running and a known load.
Review the distribution of single-phase loads and the taps of the transformer that feeds the bus.

What not to do

Do not raise the 46 pickup or disable it to keep the motor in service: an open phase will damage it without warning.
Do not restart a motor that tripped on phase loss without checking fuses and poles.
Do not change the rotation setting without verifying the physical rotation of the motor.
Do not rely only on overload 49 if the thermal model does not weight negative sequence.

How much unbalance a motor tolerates

Standards set how far a motor can operate with unbalanced voltages, and that limit is the starting point for the 46 setting. According to NEMA MG 1, a motor can operate at its rated power with a voltage unbalance of 1 % or less. Above that value a derating curve applies. Operating with more than 5 % unbalance is not recommended. The exact curve and figure number change between editions of the standard, so they are taken from the current edition and from the motor manufacturer’s data.

Internationally, IEC 60034-26 covers the effects of unbalanced voltages on three-phase cage induction motors. The second edition, published in 2026, clarifies that the voltages considered are line-to-line, and its clause 4 addresses the effects on currents, heating, torque, full-load speed and efficiency; clause 5 covers derating to avoid overheating.

The amplification between voltage and current is what surprises in the field. A commonly used technical publication cites, as an example, that with 5 % voltage unbalance, current unbalances close to 40 % can appear. The real factor depends on the motor design and should not be taken as a rule. For that reason, when the current unbalance of a bus with several motors is much higher than the measured voltage unbalance, there is no contradiction: it is the expected behavior of the negative-sequence impedance.

Two practical consequences for the setting:

A 46 pickup more sensitive than the unbalance the motor withstands according to its manufacturer will produce trips with no damage to the motor. The setting must come from the permissible unbalance of the motor and from the unbalance that really exists on the bus.
Thermal overload must recognize the extra rotor heating from negative sequence. If the relay thermal model does not weight it, the 46 is the only defense against that heating, and it is worth confirming in the relay manual how it is implemented.

An open phase is the extreme case of the same phenomenon: with the motor running, the negative-sequence current approaches the positive-sequence one and the rotor heats quickly. If the relay has a specific phase-loss or current-unbalance function with a short time, that function should act first, and the inverse-time 46 stays as backup.

When to call us

If the 46 of your motors trips and it is not clear why, send us the relay events and records: we quote the trip analysis. Our team reviews the unbalance settings, the thermal model and the rotation with remote relay configuration on the main brands. If the motor has to be tested or the grid measured, we do it with motor tests, in Colombia or elsewhere on request, and the power quality study. Write to us.

References

IEEE C37.96, Guide for AC Motor Protection.
IEC 60034-1, rotating electrical machines: rating and performance.
IEC 60034-26, effects of unbalanced voltages on the performance of three-phase cage induction motors.
NEMA MG 1, Motors and Generators.
Blackburn and Domin, Protective Relaying: Principles and Applications.

Frequently asked questions

Why does the 46 trip if the motor is fine?

Because the 46 protects the motor from what happens in its supply: an open phase, a grid voltage unbalance or a reversed connection. The motor can be healthy and still receive unbalanced currents that would heat it. The question is where the negative sequence comes from, and it almost always comes from outside the motor.

Can a small voltage unbalance make the 46 trip?

Yes. The motor impedance for negative sequence is similar to the locked-rotor one, much lower than the running impedance. A voltage unbalance of a few percent therefore produces a current unbalance several times larger. The permissible limit of each motor is given by its manufacturer with reference to standards such as IEC 60034-1 or NEMA MG 1.

What happens if a motor loses a phase while running?

If the load allows, it keeps turning on two phases and draws a much higher current through them, with high negative sequence that heats the rotor. Without a suitable unbalance or phase-loss protection, the motor overheats in a short time. When trying to start with one phase open, it does not start and stays at locked-rotor current.